With reference to FIG. 1, an LVDT (for Linear Variable Differential Transformer) sensor 1 conventionally includes a cylindrical body in which a transformer and a magnetic core 2 are integrated.
The transformer comprises a primary winding 3, a first secondary winding 4 and a second secondary winding 5. The magnetic core 2 slides axially within the cylindrical body and thus changes the distribution of the magnetic field that prevails in said cylindrical body.
An excitation voltage Ve is applied across the terminals of the primary winding 3. The excitation voltage Ve has a specific excitation period and a specific excitation frequency. A first measurement voltage Va is measured across the terminals of the first secondary winding 4 and a second measurement voltage Vb is measured across the terminals of the second secondary winding 5.
The position of the magnetic core 2 is obtained from the ratio R:R=(amp(Va)−amp(Vb))/(amp(Va)+amp(Vb)),where amp(Va) is the amplitude of the first measurement voltage Va and where amp(Vb) is the amplitude of the second measurement voltage Vb.
Generally, the first measurement voltage Va is acquired and digitized over an entire excitation period and at a sampling frequency higher than the excitation frequency, and amp(Va) is obtained by means of synchronous demodulation of the first measurement voltage Va over this excitation period. Then, during the subsequent excitation period, the second measurement voltage Vb is acquired and digitized with the same sampling frequency, and amp(Vb) is obtained by means of synchronous demodulation of the second measurement voltage Vb over one excitation period.
The ratio R is then calculated once amp(Va) and then amp(Vb) have been obtained. The response time of the measurement is therefore greater than two excitation periods, which is relatively long. However, this solution allows the acquisition to be carried out by a common part of a single acquisition chain (not including electromagnetic interference filtering components), and therefore by the same analog-to-digital converter. The gain error for the measurement is thus reduced.
It should be noted that the non-zero velocity of the magnetic core 2 means that the ratio R is not constant across the two excitation periods, and this generates a drag effect on the measurement of the position of the magnetic core 2.
Alternatively, the first measurement voltage Va and the second measurement voltage Vb are acquired and digitized simultaneously by two separate acquisition chains over the course of the same excitation period and with the same sampling frequency.
The ratio R is then calculated once amp(Va) and amp(Vb) have been simultaneously obtained, and therefore the response time of the measurement is in this case close to one excitation period. However, the gain error is increased due to the use of two separate acquisition chains for VA and VB.
Alternatively, it is also possible to use asynchronous demodulation. Asynchronous demodulation of this kind is, however, more sensitive to environmental noise, whatever the frequency thereof.